Richard what are talking about? Have you been following south african tours to the sub-continent? If you check you will notice that, for many years south africa has not won a single odi series in the sub-continent[barring one series win in pakistan].
But they have won them in West Indies, and even now West Indies is not quite the same as India and Sri Lanka.
In any case, let's have a proper look at SA's recent record in the subcontinent, in reverse order...
ICC CT in 2006\07... not that there were a great deal of games in this tourney that bore resemblence to "typical" subcontinental conditions, but SA won 2 games and lost 2 - hardly the worst performance in The World.
Bilateral series in India in 2004\05... once again, this hardly bore any resemblence to "typical" subcontinental conditions. All the pitches offered plenty to the seamers early on, and it was win-toss-win-match. 2 tosses each, 2 games each.
Bilateral series in Sri Lanka in 2004... lost 5-0, but it's worth remembering that for some reason their ODI form around then was awful. They'd lost their last 5 before that, too, in New Zealand, in conditions bearing no resemblence. They lost their next 2, too, in England and at home. Kinda suggests the subcontinent didn't really have much to do with it.
TVS Cup in Bangladesh in 2003... won 1, lost 1 to India, and would probably have won another but for rain. Pretty decent performance where I'm standing.
ICC CT in Sri Lanka in 2002\03... won 1, lost 1, and would've won the one they lost but for one of the worst pieces of stalling you'll ever see.
Singer Tri-Series in Sri Lanka in 2000... won 2, lost 3. Hardly catastrophic, especially given that it was just after Cronjegate.
Coca-Cola Cup in Sharjah in 1999\2000... won 3, lost 2. A winning margin.
Bilateral series in India in 1999\2000... won 2, lost 3. Lost, but not badly.
And their last 7 games before that, in Bangladesh in ICC KO 1998\99 and the quadrangular in 1997\98, were all victories.
So I don't really see that SA's subcontinental form is so abysmal as to write-off their chances in this World Cup.